fix: avoid zero-rate queue reader throttling

[mykaul]

Summary

• When persistenceMaxQPS is set to 0 (meaning "unlimited"), the queue reader rate limiter receives a zero rate, causing all queue readers to fail with "burst size is smaller than required token count" and never process tasks.
• This fixes the rate function to return a high default (100,000) when persistence QPS is unlimited, and adds a missing return after the error log to prevent falling through to an invalid lock/read path.

Bug Details

NewHostRateLimiterRateFn computes float64(persistenceMaxRPS()) * persistenceMaxRPSRatio. When persistenceMaxRPS() returns 0 (unlimited), this produces rate=0, which creates a rate limiter with burst=0. The reader then cannot acquire even 1 token, logging the error but continuing into r.Lock() without tasks — effectively a tight error loop.

Testing

• Verified with benchmarks that queue readers function correctly with persistenceMaxQPS: 0 in dynamic config.
• Existing unit tests pass.